I am getting OpenCL errors on the same hardware that H15.5 worked fine on.

Do I need to install OpenCL? (I have never before AFAIK)

If Houdini requires this software why doesn't the Houdini installer simply install it or at least mention that you need to run another installer.

I am pretty much dead in the water with H16 on my Windows 10 nVidia based system.

What button do I click on to make Houdini 16 work correctly?

3 things to do:

Update your Nvidia drivers to the latest

set the env variable to:
HOUDINI_OCL_DEVICETYPE=“GPU”

Download the AMD OpenCL CPU driver.

I had some issue with H16 on Windows 10 as well.

My case was really weird since I have a HD4000 and a GTX 860M on my laptop.

Houdini got kinda confused about which GPU to use for Viewport (OpenGL) and which one to use for OpenCL.

In addition to using the latest nVidia drivers, in the nVidia control panel, the defaults for 3D settings is “let application decide”. So, first thing to do is change that to “nVidia GPU” for all 3D settings.

Now, once you add the environment variable like Artye says, Houdini will ask the OS for a GPU device, and since in the nVidia control panel you configured it to use nVidia GPU's, that's the GPU that what will be provided to Houdini.

Thanks for the tips.
I tried re-installing the nVidia driver. I chose Custom and Clean install. This fixed my problem.

I can use the Terrain tools!